1. Field of the Invention
The present invention relates to a carbazole polymer for an organic electroluminescent (EL) element and a method of manufacturing the carbazole polymer.
2. Description of the Background
Organic thin-layer electroluminescent (EL) components are attractive and research into their commercialization has continued because their self-luminescence provides advantages such as superior view angle dependency and visibility as well as space savings because the organic thin-layer EL components are a thin-layer type complete solid state component. At present, however, many areas requiring improvement remain, such as energy conversion efficiency, luminescent quantum efficiency, and stability (device durability).
Until now, organic thin-layer EL components using a low molecular weight compound or a polymer have been reported. With regard to the organic thin-layer EL components using a low molecular weight compound, it is reported that efficiency is improved by employing various laminate structures and durability is improved by suitably controlling the doping method.
However, in the case of low molecular aggregations, the layer state changes over a long time, which means that the organic thin-layer EL components using a low molecular weight compound has a fundamental problem with the stability of the layer.
With regard to the organic thin-layer EL components using polymer materials, poly-p-phenylenevinylene (PPV) series, poly-thiophenes, etc. have been studied extensively. However, these materials have difficulties about improving the purity and are fundamentally low in fluorescence quantum yield. Therefore, no EL component using these materials that demonstrates high performance has been obtained yet.
Considering that the polymer materials are essentially stable in a glass state, an excellent EL component can be structured if a high fluorescence quantum yield is imparted.
In addition, with regard to the layer forming method, while a vacuum deposition method is generally used for the low molecular weight compound case, a coating method is used for the polymer material, which is preferable in terms of cost. As described above, each has its own merits and demerits.
In addition, improvement of the efficiency using triplet exciton has been studied extensively (for example, refer to T. Tsutsui et. al. JPN. J. Appl. Phys. Vol. 38 L1502, published in 1999) and it is found that the luminance efficiency can be significantly improved. Accordingly, the number of reports on host materials for use in the luminous layer has been increasing. Among these, a representative example is 4,4′-bis(carbazolyl-9)biphenyl (CBP) represented by the following chemical structure.

Based on follow-up studies, however, it became clear that crystallization of CBP proceeds in the luminous layer, thereby shortening the life of the device (for example, refer to WO 01/72927-A1).
To avoid this problem, other carbazole compounds instead of CBP have been studied (for example, refer to Japanese patent application number 2005-158691 (JP-2005-158691-A).
In addition, polymer materials having a carbazole structure have been studied (for examples, refer to JP-2004-339432-A and JP-2007-262219-A). However, all of the compounds described therein have insufficient durability and luminance efficiency.